1. Field of the Invention
The present invention relates to a NO.sub.x gas sensor having a WO.sub.3 sensing film and a fabrication method thereof, and in particular which is fabricated in a thin film form so as to sense the density of NO.sub.x gas accurately by a semiconductor type based on a variation of an electrical conductivity.
2. Description of the Conventional Art
In the past, smokes from factory and heating fuel were the main cause of air pollution. Recently, gases exhausted from vehicles has been a serious cause of air pollution. Among the gases exhausted from vehicles, HC, CO and O.sub.x are harmful to humans. A photovolataic decomposition of NO.sub.x by a solar irradiation generated ozone(o.sub.3), which causes global warming and causes acid rain.
There are two kinds of NO.sub.x gases, one is exhausted from vehicles and the other exists naturally in the air. Since NO.sub.x gas is very harmful to the human beings, the American Conference of Governmental Industrial Hygienists(ACGIH)set a threshold limit value-time weighted average (TLV-TWA), which is the recommended reference value of NO.sub.x gas for the safety of human being, up to 25 ppm of NO and up to 3 ppm of NO.sub.2.
Therefore, it is necessary to detect accurately NO.sub.x gas of low density more than 1 ppm. The conventional NO.sub.x gas density measuring methods employed an electrochemical sensor, a mass spectrometry, etc.
The conventional NO.sub.x gas sensor, however, has disadvantages in that it is difficult to install the gas sensor consistently in a portion of the unit which generates gases, such as in a vehicle, and it is difficult to miniaturize the gas sensor so as to transport the gas sensor easily to a position where the gas generation unit is placed. In addition, the fabrication cost thereof is high.
The semiconductor type gas sensor, apart from an electrochemical sensor and a mass spectrometry using an expensive apparatus and analyzer, utilizes an oxide semiconductor thin film. When gaseous molecules are adsorbed on a surface of the thin film, a transfer of electrons occurs between the semiconductor thin film and the adsorbed molecules. When an electron receiving gases, i.e., which is an oxidizing gases such as NO.sub.x gas are adsorbed, a carrier density of electron in the oxide semiconductor thin film is decreased and thus decreases an electrical conductivity thereof to increase the resistance value thereof. Therefore, the sensor, using an oxide (such as WO.sub.3) thin film which has a n-type oxide semiconducting characteristics, is an excellent sensor which shows a sharp increase of resistance because of the electrical conductivity variation characteristics (.sigma..varies.P.sub.o2.sup.-1/2) in accordance with an oxygen partial pressure variation and a phenomenon of a reduction of the electrical conductivity at the time of adsorption of the oxidizing gases on the WO.sub.3 thin film surface thereof.
Thus, the semiconductor type gas sensor has an excellent durability and also makes it possible to minimize the gas sensors.
However, the efficiency of gas adsorption must be optimized in order to use the variation of resistance as mentioned above. Thus, the sensing film should be held at an appropriate temperature, and this requires a heater placed around a portion of the sensing film in order to heat the film. Miniaturization is an advantage of the semiconductor type gas sensor, however, the emitting heat must be minimized because heat affects the life span of batteries of a portable gas sensor. Also, it is important to control the temperature of the sensor varied depending on gas flow in order to accurately measure the amount of gases in the portion in which there exists a flow of vehicle's exhausted gases.
Furthermore, the conventional semiconductor type gas sensor has disadvantages in that unless the electron transfer occurs rapidly when NO.sub.x gas is adsorbed on the surface of the sensing thin film it is difficult to accurately detect amount of gas, and an extended waiting time is required for repetitive measurements.